1. Field of the Invention
The present invention relates to a carrier that frictionally charges a toner, a two-component developer including at least the toner and carrier, and an image forming apparatus using the developer.
2. Discussion of the Background
In a conventional electrophotographic image forming method, an electrostatic latent image is formed on an image bearer formed of a photoconductive material, etc. and a charged toner adheres to the electrostatic latent image to visualize the latent image. After the visualized image with the toner is transferred onto a transfer medium, such as paper, it is fixed on the transfer medium with heat, pressure, or a solvent gas to become a produced image.
The methods of charging toner are broadly classified as (1) a two-component developing method wherein the toner and carrier are stirred and mixed to charge the toner; and (2) a one-component developing method wherein the toner is charged without the carrier. Further, whether magnetism is used to bear the toner particles on a developing roller classifies the one-component developing method to a magnetic one-component developing method or a non-magnetic one-component developing method.
The two-component developing method has been used in printers, copiers, and other complex machines requiring high-speed printing and good image reproducibility to comply with demands for charging stability and rising edge of the toner, and long-term stability of the resultant image quality. On the other hand, the one-component developing method is mostly used in small printers and facsimiles requiring space and cost savings.
Particularly in these days, colorization of images makes progress and demands for high quality images and stability of image quality are increasing.
Japanese Laid-Open Patent Publication No. 58-184157 and Japanese Patent Publication No. 5-8424 describe a two-component developing method using a magnetic carrier, wherein the carrier has a small particle diameter in accordance with a small particle diameter of the toner and more exquisite fuzz of a developer brush on a magnetic sleeve (developing sleeve) to develop a higher quality latent image.
However, when the magnetic carrier has a small particle diameter, magnetization per one carrier particle is small and a magnetic binding force thereof onto the magnetic sleeve becomes small. Therefore, carrier transfer, i.e., carrier adhesion onto the image bearer occasionally occurs.
To prevent carrier adhesion resulting from the small particle diameter of the magnetic carrier, in developing methods of feeding a developer by rotating a magnet included in a developing sleeve, Japanese Laid-Open Patent Publication No. 2000-137352 describes a method of setting a lower limit of the carrier saturation magnetization, and Japanese Laid-Open Patent Publication No. 2000-338708 describes a method of setting a lower limit of a product between a particle diameter and a residual magnetization of the magnetic carrier.
In other words, these methods prevent feeding the carrier having a small magnetic binding force before the feeding occurs. However, as an electrostatic element is added to the carrier in the image developer, a desorption force thereof is occasionally higher than the binding force and carrier adhesion cannot sufficiently be prevented.
In Japanese Laid-Open Patent Publication No. 2000-137352, a saturation magnetization in an electric field of 10,000 Oe is used. However, such a high electric field is not used in conventional electrophotographic image developers, and carrier adhesion cannot always be sufficiently prevented even when this method is used.
Japanese Laid-Open Patent Publication No. 4-145451 describes a method of removing carrier particles having a specific low saturation magnetization, a small particle diameter, and a small specific gravity, regardless of their particle diameters to prevent carrier adhesion. However, in Japanese Laid-Open Patent Publication No. 4-145451, the final properties of the carrier are not clarified at all, and sufficient prevention of carrier adhesion cannot be expected at present when further uniformity of the carrier particles is demanded.
Japanese Laid-Open Patent Publication No. 2002-296846 describes a method of specifying a volume-average particle diameter, a particle diameter distribution, an average airspace particle, a magnetization in a magnetic filed of 1,000 Oe of a core material of a carrier, and a magnetization difference between the carrier and scattered materials to prevent carrier adhesion. It can be supposed that the method of Japanese Laid-Open Patent Publication No. 2002-296846 has a specific prevention effect for the carrier adhesion because it prevents the presence of particles having a small magnetic binding force. However, as carrier adhesion depends on not only a magnetic binding force but also on a balance between the magnetic binding force and a sum of a mechanical and an electrostatic desorption force of the carrier particles, carrier adhesion occurs according to developing conditions only when the magnetic binding force is controlled. When a carrier having a comparatively large particle diameter is used (so as not to substantially include particles having a particle diameter greater than 12 μm), a developer brush formed on the developing sleeve (magnetic sleeve) remains fixed as it is for a long time.
Further, in Japanese Laid-Open Patent Publication No. 2002-296846, properties of a carrier core material are controlled to prevent the carrier adhesion and have other effects. However, as the carrier properties largely depend on mechanical, chemical, electrical, physical, and thermal properties of a coated layer of the carrier besides the properties of the core material, controlling only the core material properties does not always sufficiently control the carrier properties. Particularly, as image quality and stability thereof largely depend on properties of carrier surface when actually used in an image forming apparatus, carrier particles having a coated layer need to be used for better image quality.
Recently, in consideration of environmental protection, units using one-component developing method are mostly recycled and reused. At the same time, two-component developers are required to have higher longevities.
On the other hand, for decreasing energy consumption, a toner image fixing temperature is further decreasing, and the toner is easily deformed and firmly fixed at a lower temperature.
The two-component developers deteriorate because of: (1) carrier surface abrasion; (2) separation of a coated layer on the carrier surface; (3) carrier crush; and (4) deterioration of the chargeability, transfer from a desired electric resistance of the carrier, and generation of foreign particles such as broken pieces and abrasion powders accompanied by fixation (spent) of a toner on the carrier. These cause image quality deterioration, such as deterioration of image density, foggy background, and deterioration of image resolution. Deterioration, such as occurrence of physical and electrical damage of the image bearers, also occurs.
Many suggestions having some benefit have been made to solve the above-mentioned problems and to improve durability of the carrier.
As one suggestion paying attention to a coated layer of a coated carrier, i.e., a carrier having a coated layer on a surface of its core material, Japanese Laid-Open Patent Publication No. 8-6308 describes a carrier having a coated layer which is a hardened polyimide varnish including specific bimaleimide to improve stability against environment, and to prevent foggy background and separation of the coated layer. As another proposal, Japanese Patent No. 2998633 describes a carrier having a resin coated layer wherein a matrix resin includes dispersed resin particles and electroconductive fine particles to prevent the toner from becoming spent for a long time. Japanese Laid-Open Patent Publication No. 9-311504 describes a carrier having a coated layer formed of a phenol resin including a hardened amino group on a surface of a spheric complex core particulate material formed of an iron oxide powder and a phenol resin, wherein contents of the iron oxide powder and the amino group are specified to obtain a stable frictional charge and durability. Japanese Laid-Open Patent Publication No. 10-198078 describes a carrier having a coated layer formed of a matrix resin including dispersed resin fine particles and electroconductive fine particles, wherein the matrix resin includes not less than 10% of components of a binder resin of the toner to decrease an influence of the remaining spent toner on the chargeability of the carrier. Finally, Japanese Laid-Open Patent Publication No. 10-239913 describes a carrier having a coated layer formed of a polyimide resin having a repetition group including a diorganosiloxy group and a compound including two or more epoxy groups in a molecule to have a stable charged amount.
However, these suggestions do not achieve sufficient effects at present when the fixing temperature further decreases and higher longevity of the carrier is expected. In Japanese Laid-Open Patent Publication No. 8-6308, Japanese Patent No. 2998633 and Japanese Laid-Open Patent Publication Nos. 9-311504 and 10-239913, the matrix resin occupies most of the carrier surface alone and the toner fixation mostly depends on the surface status of the matrix resin. Therefore, sufficient improvement of preventing toner from becoming spent is not achieved. In Japanese Laid-Open Patent Publication No. 10-198078, when a toner having a low temperature fixability is used, components on the surface of the carrier that are the same as those of the toner binder resin tend to be a base point of the toner fixation, and the toner is occasionally not stably charged from the beginning of the toner agitation.
Many suggestions of forming a coated layer with a silicone resin having comparatively a low surface energy have also been made. However, the silicone resin has a problem of deficient adherence to a core material of the carrier due to the low surface energy.
Japanese Laid-Open Patent Publication No. 58-108548 describes a carrier coated with a specific resin; Japanese Laid-Open Patent Publications Nos. 57-40267, 58-108549, 59-166968 and 6-202381 and Japanese Patent Publication No. 1-19584 describe carriers coated with specific resins including various additives; and Japanese Patent No. 3120460 describes a carrier coated with the specific resin and an additive is adhered on the surface thereof. Japanese Laid-Open Patent Publication No. 8-6307 describes a carrier mainly coated with a benzoguanamine-n-butylalcohol-formaldehyde copolymer. Japanese Patent No. 2683624 describes a carrier coated with a cross-linked resin between a melamine resin and an acrylic resin. However, these carriers do not yet have sufficient durability.
To improve charged amount instability of the carrier accompanied by the spent toner on the surface thereof and resistance variation due to an abrasion of the coated resin, Japanese Laid-Open Patent Publications Nos. 2001-117287, 2001-117288 and 2001-188388 describe a carrier coated with a thermoplastic resin and a carrier coated with the thermoplastic resin having a larger particle diameter than that of the binder resin.
Japanese Laid-Open Patent Publication No. 9-319161 describes a method of dispersing fine particles of a specific thermoplastic resin in the matrix resin of the coated layer as another method of maintaining the coated layer properties of the carrier, particularly the chargeability thereof. By this method, even an abraded coated layer has equivalent properties to those of the initial coated layer. However, the method does not sufficiently decrease abrasion.
Even the method in Japanese Patent No. 2998933, wherein an electroconductive fine powder is dispersed at the same time in addition to the specific thermoplastic resin, does not sufficiently decrease the abrasion.
As mentioned above, trials of fundamentally improving the carrier adhesion in a two-component developer expected to produce high quality images in order to stably produce high quality images have not been made so far with a concept that various binding forces and desorption forces applied to the carrier particles in image developers should be within proper ranges, and this still remains a difficult problem. Further, preventing carrier adhesion and abundantly and softly forming or properly renewing a developer brush on a developing sleeve to properly feed the toner onto an electrostatic latent image bearer and produce high quality images with a high density and without background fouling still remain a difficult problem.
Because of these reasons, a need exists for a carrier producing high quality images without carrier adhesion and having good durability.